FIG. 1 and FIG. 2 depict a prior art pressure sensor 10 mounted on a flexible plastic substrate 16. The sensor 10 and the substrate 16 are both enclosed within a protective flexible plastic sleeve 11. The sensor 10 is used to detect or sense a person lying in a bed or sitting in a chair. The sleeve 11 protects the sensor 10, especially from liquids.
The sensor 10 is comprised of two, thin, flat and elongated conductive panels 12 and 14. The panels 12 and 14 are kept spaced apart from each other by non-conductive spacers 13 and 15 applied to one or both of the sides of the panels 12 and 14 that face each other. Each of the panels 12 and 14 has a top side 12A and 14A and a bottom side, 12B and 14B. The spacers 13 and 15 are embodied as thin, narrow and elongated, adhesive-backed non-conductive strips that run along the side edges of a panel 12 and 14.
In FIG. 2, the top panel 12 is shown having two spacers that are identified by reference numeral 13 and which are attached to the lower face 12B of the top panel 12. The lower or bottom panel 14 is shown as having two spacers 15 that run along the side edges of the upper surface 14A of the bottom panel 14. Both sides of the spacers 13 and 15 have an adhesive backing, to affix the corresponding spacer to the surface of a panel.
The spacers 13 and/or 15 hold the panels 12 and 14 away from each other by a fixed distance that is equal to the spacer thickness. The spacers thus electrically isolate or separate the panels 12 and 14 from each other.
As can be seen in FIG. 2, an elongated fulcrum wire 20 is located between the second side 14B of the lower panel 14 and a relatively stiff, non-conductive switch-supporting substrate 16. The fulcrum wire 20 is centered or substantially centered between the two long, side edges of the lower panel 14 and runs almost the entire length of the lower panel 14.
When a force is applied to the top panel 12 either directly or through the envelope 11, the applied force will cause the panels 12 and 14 to bend around the fulcrum wire 20. Bending the panels 12 and 14 around the fulcrum wire 20 causes the outside edges of the panels to deflect downward toward the substrate 16, effectively causing the top surface 14A of the lower panel 14 to approach and eventually make contact with the bottom surface 12B of the upper panel 12. Bending the panels 12 and 14 around the fulcrum wire 20 thus eventually causes the bottom panel 14 to make a direct connection with the conductive top panel 12.
When the top panel 12 and the bottom panel 14 electrically contact each other in response to a pressure or force applied to the top panel, the structure shown in FIG. 2 functions as a switch. The switch/sensor 10 “closes” when a force sufficient to bend the panels 12 and 14 around the fulcrum wire 20 causes the two panels to make an electrical connection between them. The switch/sensor 10 opens when a switch-closing force is removed.
The amount of force required to close the switch/sensor 10 will depend on several different physical factors that include the width and thickness of each of the panels 12 and 14 as well as the material from which the panels 12 and 14 are fabricated. The thickness and width of the spacers 13 and/or 15 will also affect or determine the amount of pressure required to close the switch. Finally, the diameter and construction of the fulcrum wire 20 will also affect or determine the amount of pressure required to deform or bend the panels 12 and 14 such that they make electrical contact to each other. Experimentation has empirically determined the parameters of switches that will “close” at specific pressures.
It should be understood that as used herein, the term “sensor” refers to a two-state pressure-responsive switch which closes in response to an applied force, the required magnitude of which depends on one or more of the factors mentioned above. The terms “sensor” and “pressure sensor” should not be confused with a transducer, which is considered herein to be a device that generates, creates or outputs a signal representative of a measurable electrical characteristic of a sensor and/or the panels it is constructed from.
While the prior art sensors depicted in FIG. 1 and FIG. 2 have proven to be functionally adequate for detecting the presence or absence of a person on a mattress or sitting in a chair, they are somewhat deficient in their ability to work with patient monitoring systems that provide patient data to remotely located health care providers or patient monitors.
The prior art sensor 10 depicted in FIG. 1 and FIG. 2 tends to break down over time because of the environments in which they are typically used. Water, cleaning solutions and body fluids often leak into the sleeve 11, or the sleeve is punctured, the result of which is short-circuiting of the conductive panels 12 and 14. Electrical connections between the panels 12 and 14 and an external connector which runs through a cable 22 can also corrode. Providing some electronic intelligence to the sensor 10 that would enable the sensor 10 to be more reliable for use with remote or wireless patient monitoring systems and which might enable testing the sensor's functionality would be an improvement over the prior art.